Turbojet Axial Rotor Thrust - SR-71
By the way, something I can contribute that I believe is novel is not novel but in approach.
The improvement to the engine was to eliminate in the turbo jet that which is necessary to create a Ramjet, The “choke”
There is no Ramjet without choke, and there is no turbojet with a choke.
The displacement of the “problem” was not effective in increasing thrust overall, but in maximizing the turbojet’s performance, no? That is (was) not new.
Lockheed, functionally is a division of the government. It relies on public resources to pay for its work and profit.
The whole “Ramjet” thing was marketing. In marketing, it’s called “puffing” (no pun intended). Pilots, like myself, took it in and preferred rather to be impressed than skeptical....
The improvement to the engine was to eliminate in the turbo jet that which is necessary to create a Ramjet, The “choke”
There is no Ramjet without choke, and there is no turbojet with a choke.
The displacement of the “problem” was not effective in increasing thrust overall, but in maximizing the turbojet’s performance, no? That is (was) not new.
Lockheed, functionally is a division of the government. It relies on public resources to pay for its work and profit.
The whole “Ramjet” thing was marketing. In marketing, it’s called “puffing” (no pun intended). Pilots, like myself, took it in and preferred rather to be impressed than skeptical....
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With respect, how do the following statements "increase your knowledge" and "answer your classroom rubric":
Lockheed, functionally is a division of the government. It relies on public resources to pay for its work and profit......The whole “Ramjet” thing was marketing.
The above appears to be more political statements than technical statements and have little if anything to do with "increasing your knowledge."
Or did I miss something?
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Back To The Original Questions
megan,
I assume the "at cruise" to be above Mach 3 in which case most of the compression is coming from the inlet. I assume that was what Graham was conveying.
Axial loading on rotors do reverse with thrust settings, acceleration (pouring on the coals) and deceleration (hitting the brakes). These changes have to be accounted for in the design of the rotor components. The rotors even move slightly in the axial direction forward and aft as a result of acceleration or deceleration, so spacing between the stationary vanes and rotors has to be calculated plus a safety margin to prevent clashing between rotors and stators in either direction.
Thrust bearings are of a ball bearing design of which there are several types. They are designed to be contact as uniformly as possible and never free floating. Roller bearings are not used in the axial direction, but are used in some situations in a radial direction.
A reading of "SR-71 Revealed" by Richard Graham, pilot and the 9th Strategic Reconnaissance Wing Commander, he says, "at cruise, the rotor of the engine actually has a small negative thrust load on the engine".
I'm trying to understand the import of his statement.
In normal operational circumstances of a turbojet I'd expect a fairly large axial drag component on the rotor (negative thrust if you like, since the turbine is driving the compressor). Without identifying the engine or thrust capability, a NASA report cites a axial load of 3,000lb on a medium size engine. The engine at SR-71 cruise provides 17% of the thrust, the rest being 54% from the inlet and 29% from the ejector..
Is this merely indicative that most of the compression is coming from the inlet rather than the compressor?
I've seen statement that the axial loading of a turbojet rotor can reverse direction depending on circumstance, such as RPM. Anyone with insight?
I'm trying to understand the import of his statement.
In normal operational circumstances of a turbojet I'd expect a fairly large axial drag component on the rotor (negative thrust if you like, since the turbine is driving the compressor). Without identifying the engine or thrust capability, a NASA report cites a axial load of 3,000lb on a medium size engine. The engine at SR-71 cruise provides 17% of the thrust, the rest being 54% from the inlet and 29% from the ejector..
Is this merely indicative that most of the compression is coming from the inlet rather than the compressor?
I've seen statement that the axial loading of a turbojet rotor can reverse direction depending on circumstance, such as RPM. Anyone with insight?
Axial loading on rotors do reverse with thrust settings, acceleration (pouring on the coals) and deceleration (hitting the brakes). These changes have to be accounted for in the design of the rotor components. The rotors even move slightly in the axial direction forward and aft as a result of acceleration or deceleration, so spacing between the stationary vanes and rotors has to be calculated plus a safety margin to prevent clashing between rotors and stators in either direction.
Thrust bearings are of a ball bearing design of which there are several types. They are designed to be contact as uniformly as possible and never free floating. Roller bearings are not used in the axial direction, but are used in some situations in a radial direction.
Float
megan,
I assume the "at cruise" to be above Mach 3 in which case most of the compression is coming from the inlet. I assume that was what Graham was conveying.
Axial loading on rotors do reverse with thrust settings, acceleration (pouring on the coals) and deceleration (hitting the brakes). These changes have to be accounted for in the design of the rotor components. The rotors even move slightly in the axial direction forward and aft as a result of acceleration or deceleration, so spacing between the stationary vanes and rotors has to be calculated plus a safety margin to prevent clashing between rotors and stators in either direction.
Thrust bearings are of a ball bearing design of which there are several types. They are designed to be contact as uniformly as possible and never free floating. Roller bearings are not used in the axial direction, but are used in some situations in a radial direction.
I assume the "at cruise" to be above Mach 3 in which case most of the compression is coming from the inlet. I assume that was what Graham was conveying.
Axial loading on rotors do reverse with thrust settings, acceleration (pouring on the coals) and deceleration (hitting the brakes). These changes have to be accounted for in the design of the rotor components. The rotors even move slightly in the axial direction forward and aft as a result of acceleration or deceleration, so spacing between the stationary vanes and rotors has to be calculated plus a safety margin to prevent clashing between rotors and stators in either direction.
Thrust bearings are of a ball bearing design of which there are several types. They are designed to be contact as uniformly as possible and never free floating. Roller bearings are not used in the axial direction, but are used in some situations in a radial direction.
2. This movement is precipitated by acceleration and deceleration.
3. If there is a constant positive flow against the turbine, there will be no float.
4. The turbine is parasitic, removing only enough power to drive the compressor, by design.
5. If at any time the inlet “overpowers” the compressor, by eliminating the “suck”,
6. The drive is overpowered by the inlet, the parasitic turbine is “removed” from the flow now either in free stream,
7. Or in “compression’, now actually providing thrust, rather than removing it.
8. The net thrust (rotor) is forward.
9. The thrust (on the rotor) has been reversed, and the bearings must have unloaded, and are now loaded in the opposite direction.
OR,
Colonel Graham is mistaken, thinking a reduction in positive thrust amounts to a reversal into the (net) negative.
No?
Do you have an analysis of unstart? Seems to me this system is extremely sensitive to variations in thrust, and homogeneity of inlet compression?
So, in my simple mind, I want to know if Graham is talking about a “reduction” in thrust that causes “negative” only in “net”? Or, an actual reversal of vector of net rotor thrust, a reversal.
If the rotor is pushed into the aft face of the forward bearing, instead of the usual forward face of the aft bearing, as the result of a reversal, then by definition there is “float” between faces, however undesirable that may be?
Bear in mind, the rotor is expected to be independent of the gas path as a whole, by definition as to variation of gas path flow. Within the limits of the thrust bearings.
It seems to me this reversal is not and should not be “catastrophic”. It merely means that for brief periods, the function of the rotor “flips”; it still remains in the gas path, its relative thrust doesn’t effect any substantial changes in the engine’s performance. It is the inlet that is the critical player.
You claim there is movement between stators (attached to the case) and blades of the rotor.
Stopping the rotor short of clash is the job of both thrust bearings. You know there have to be at least two? Unless the design requires one bearing to protect both directions of thrust?
imo
Last edited by Concours77; 13th Sep 2018 at 15:45.
Cocours77
Are you postulating, stating facts or asking a question ?
Without a SR-71 J58 design manual at hand I can't state any reliable facts. I participate only from the standpoint of design theory.
Are you postulating, stating facts or asking a question ?
Without a SR-71 J58 design manual at hand I can't state any reliable facts. I participate only from the standpoint of design theory.
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ref Megan's original question :
"at cruise, the rotor of the engine actually has a small negative thrust load on the engine" from Richard Graham's book . He was obviously repeating an explanation from someone else, ie would have been second or third hand, perhaps with a bit of miscommunication thrown in. Ended up confusing rotor thrust with engine thrust, which some people do. They say, big fan engine for example, most of the engine thrust comes from the fan but then presume it comes through the fan rotor thrust bearing. Except airfoil aerodynamic load contributions to total rotor load are not necessarily the biggest nor in the same direction. More to the point FWIHR J58 bearing skidding only occurred during windmilling . Also the follow-on supersonic cruise engine, JTF17, design had to be based heavily on previous high mach experience , ie J58, and would have had 4,000 lb forward at M3, 70,000 ft.
So, should he really have said engine load was rearwards? in which case he would have been repeating what David Campbell says in next line. He was Ben Rich's man on propulsion system and inlet patent holder.
"at M3+ if afterburner reduced to minimum engine would be dragging on engine mounts" ie t/mc was actually a drag item and needed more than min a/b to provide thrust through mounts.
The effect of a/b setting from an internal thrust imbalance pov is given by "converging engine nozzle is a drag item. Afterburning allows nozzle to be opened up which reduces rearward load and this is the origin of thrust boost, ie internal thrust out-of-balance ( = engine thrust) now has a reduction in one of the rearward contributions" RR book
Or conversely closing down nozzle, as in first line, increases nozzle drag so engine is now dragging on mounts.
We would expect a turbojet to run out of thrust when reaching a certain speed, on paper that is, "for a turbojet engine, depending on compressor pressure ratio, ram recovery and turbine inlet temperature, thrust drops to zero at M3+" NACA performance study. ie needs an afterburner
And it looks like the J58 t/mc had already ceased to be a thrust producer at cruise (not withstanding its bleed-tube-enabled increase in compressor mass flow and hence thrust) in so far as its pressure loss meant it didn't contribute to a/b nozzle pressure ratio "J58 t/mc pr 0.9 at cruise" ( P&W engine/nacelle total pressure schematic avail at enginehistory.org)
"at cruise, the rotor of the engine actually has a small negative thrust load on the engine" from Richard Graham's book . He was obviously repeating an explanation from someone else, ie would have been second or third hand, perhaps with a bit of miscommunication thrown in. Ended up confusing rotor thrust with engine thrust, which some people do. They say, big fan engine for example, most of the engine thrust comes from the fan but then presume it comes through the fan rotor thrust bearing. Except airfoil aerodynamic load contributions to total rotor load are not necessarily the biggest nor in the same direction. More to the point FWIHR J58 bearing skidding only occurred during windmilling . Also the follow-on supersonic cruise engine, JTF17, design had to be based heavily on previous high mach experience , ie J58, and would have had 4,000 lb forward at M3, 70,000 ft.
So, should he really have said engine load was rearwards? in which case he would have been repeating what David Campbell says in next line. He was Ben Rich's man on propulsion system and inlet patent holder.
"at M3+ if afterburner reduced to minimum engine would be dragging on engine mounts" ie t/mc was actually a drag item and needed more than min a/b to provide thrust through mounts.
The effect of a/b setting from an internal thrust imbalance pov is given by "converging engine nozzle is a drag item. Afterburning allows nozzle to be opened up which reduces rearward load and this is the origin of thrust boost, ie internal thrust out-of-balance ( = engine thrust) now has a reduction in one of the rearward contributions" RR book
Or conversely closing down nozzle, as in first line, increases nozzle drag so engine is now dragging on mounts.
We would expect a turbojet to run out of thrust when reaching a certain speed, on paper that is, "for a turbojet engine, depending on compressor pressure ratio, ram recovery and turbine inlet temperature, thrust drops to zero at M3+" NACA performance study. ie needs an afterburner
And it looks like the J58 t/mc had already ceased to be a thrust producer at cruise (not withstanding its bleed-tube-enabled increase in compressor mass flow and hence thrust) in so far as its pressure loss meant it didn't contribute to a/b nozzle pressure ratio "J58 t/mc pr 0.9 at cruise" ( P&W engine/nacelle total pressure schematic avail at enginehistory.org)
Last edited by PeterKent; 14th Feb 2020 at 16:02. Reason: clarifications
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Last edited by Gauges and Dials; 14th Feb 2020 at 06:00.
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The pressure recovered from the inlet, sending the air through the bypass tubes directly to the afterburner, "provides" the air for the thrust, in the same way the compressor "provides" the air for the thrust in the engine core or the fan "provides" the thrust in a fanjet.
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The pressure recovered from the inlet, sending the air through the bypass tubes directly to the afterburner, "provides" the air for the thrust, in the same way the compressor "provides" the air for the thrust in the engine core or the fan "provides" the thrust in a fanjet.
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Think of it this way - at cruise Mach numbers, the engine itself doesn't do much - it's basically there to generate the airflow through the inlet/exhaust. Ramjets don't work at lower Mach numbers - much below ~Mach 2 there simply isn't enough ram compression - hence the need for the turbojet engine. However, the at Mach 3+, you no longer need the turbojet, and pure Ramjets/Scramjets become quite efficient above Mach 3 (really impressive Thrust Specific Fuel Consumption - TSFC). At least in theory, the SR-71 could get better cruise fuel burn if they put some doors in that shutoff the inlet to the turbojet at cruise, let all the airflow bypass the core, and use duct burning as a pure Ramjet. Given the people who designed the SR-71 were no dummies, I assume they determined that the weight/space/complexity of closing off the turbojet inlet wasn't worth the potential TSFC gain. Hence the hybrid turbojet/ramjet system.
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Yes. They had an existing engine which fitted in the allowed nacelle diameter. The challenge was modifying the existing engine with least carve-up possible and still give what was required, hence no concentric bypass duct instead of 6 tubes, for example. Never mind a huge duct bypassing the whole turbomachine which would have had to pass more than five times as much air as the tubes. For insight into other paper-only options for "the problem/challenge" to get a J58 to do M3.2 , eg GE-style variable stators, see U S patent 3,344,606.
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I understand that if you did force analysis or put load sensors on the shafts and bearings, you'd see that the fan and compressor rotors are trying to pull the airplane forward, and the turbine rotors are trying to pull it backwards. And if you did the same thing for the stationary parts of the engine, you'd see that the forward walls of he combustion chambers and the compressor stators were trying to push the airplane forward, and the aft walls of the combustion chambers and the turbine stators were trying to push the airplane backward. But the thrust all ultimately comes from the expanding fuel-air mixture being burned. The compressor consumes one form of energy -- the rotation of the shaft -- and produces another -- compression of the air. I guess similarly the inlet nozzle in the J58 consumes one form of energy -- the forward motion of the airframe through the air -- and produces another -- compression of the air. But it doesn't really "produce thrust" any more than a ram air intake "produces power".
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The inlet feels (as we would if trying to stop it moving) a thrust or drag force because air is flowing through it. It doesn't matter why the air is moving. The force is still there. Air can flow through it in different scenarios all of which use a different energy source. On the front of a running engine air flows because fuel is burned. or if flamed out because the aircraft is losing altitude. When being tested in a wind tunnel air flows because an electric motor spins.
Nothing consumes energy , it converts it to another form. For an inlet the free stream has a certain amount of energy because it's "coming at" the engine with speed. The idea is to convert speed to pressure because that's the reason for the inlet. It's a better compressor than no inlet. (The compression with no inlet "just happens" as opposed to being manipulated by careful inlet design and actually occurs when an inlet unstarts). However some proportion of the "speed" ends up as thermal energy and hence we hear "the inlet has 80% pressure recovery". It only managed to get 80% of the "speed" to supercharge the engine compressor. And when the inlet unstarts it's doing nothing special as shown by a pressure recovery of, say, 40%.
The energy conversion above is exactly what happens in the spinning compressor. Instead of being given speeding air it has to make its own, stage by stage. The rotor speeds it up and the stator converts as much as it can to pressure.
they did have IBM punch cards and Freiden calculators that you could program a task to play pseudo music until the machine crashed. Also the early wet copiers to cut down on carbon copies and of course lots of test points from running engines. We also had "wang" calculators that I could program with punch cards to solve open 3 variables.I saw every part of this
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Think of it this way - at cruise Mach numbers, the engine itself doesn't do much - it's basically there to generate the airflow through the inlet/exhaust. Ramjets don't work at lower Mach numbers - much below ~Mach 2 there simply isn't enough ram compression - hence the need for the turbojet engine. However, the at Mach 3+, you no longer need the turbojet, and pure Ramjets/Scramjets become quite efficient above Mach 3 (really impressive Thrust Specific Fuel Consumption - TSFC). At least in theory, the SR-71 could get better cruise fuel burn if they put some doors in that shutoff the inlet to the turbojet at cruise, let all the airflow bypass the core, and use duct burning as a pure Ramjet. Given the people who designed the SR-71 were no dummies, I assume they determined that the weight/space/complexity of closing off the turbojet inlet wasn't worth the potential TSFC gain. Hence the hybrid turbojet/ramjet system.
